1. Field of the Invention
The present invention relates to a hydraulic braking system for use in an automotive vehicle, and more particularly to a hydraulic braking system for providing an anti-lock braking control mode to prevent a wheel from being locked, a traction control mode to prevent the wheel from slipping, an automatic braking mode, or the like.
2. Description of the Prior Art
In general, an automotive vehicle has four wheels of a front-left wheel, a front-right wheel, a rear-left wheel and a rear-right wheel, on each of which a wheel cylinder is operatively mounted. And, a hydraulic braking system for the vehicle is provided with a master cylinder for generating a master cylinder pressure in response to depression of a brake pedal, and a reservoir for supplying a brake fluid to the master cylinder. The master cylinder has two pressure chambers defined therein for supplying the pressure to one circuit of the front-left wheel and the front-right wheel, and the other circuit of the rear-left wheel and the rear-right wheel, or supplying the pressure to one circuit of the front-left wheel and the rear-right wheel, and the other circuit of the front-right wheel and the rear-left wheel. It is known that a control apparatus is provided between a master cylinder and a wheel cylinder to provide an anti-lock braking mode, as disclosed in Japanese Patent Laid-open publication No. 64-74153, for example. The apparatus is provided with a traction control changeover valve of a three-port and two-position type which is provided between the master cylinder and an anti-lock braking device. With the traction control changeover valve operated, when the brake pedal is depressed, the master cylinder communicates with the wheel cylinder through an anti-lock braking control valve. Therefore, a master cylinder pressure is supplied to the wheel cylinder through the anti-lock braking control valve, by which the anti-lock braking control is performed. When the brake pedal is not depressed, the master cylinder is connected to the wheel cylinder through a pump and the anti-lock braking control valve. Therefore, with the pump actuated, the pressure may be supplied from the pump to the wheel cylinder to enable the traction control, even if the brake pedal is not depressed.
In the hydraulic braking system as disclosed in Japanese Patent Laid-open publication No. 63-110064, a shut-off valve is provided between a reservoir and a master cylinder, and a relief valve is provided for blocking the communication between an inlet of the pump and the reservoir. In the traction control mode, the shut-off valve is actuated to block the communication between the master cylinder and the reservoir. In this condition, when the pump is driven to rotate, the brake fluid is supplied from the reservoir to the anti-lock braking control valve through the relief valve, so that the wheel may be braked, with the anti-lock braking control valve operated, even if the brake pedal is not depressed.
According to the above-described braking force control apparatus, however, in the case where the traction control, the automatic braking control or the like is performed, if the number of circuits with the anti-lock braking control devices is increased, the number of valves provided between the anti-lock braking control devices and the reservoir is increased to raise a manufacturing cost of the apparatus, at least.
Further, in the case where the wheel cylinders of more than two circuits are to be controlled, the brake fluid in each circuit must be pressurized by the pump. Generally, such a pump as the one employed in an anti-lock braking device of a recirculation type, is provided for feeding back the brake fluid stored in drain passages to the pressure chamber, so that its suction performance is not necessarily required to be high. In the case where the pump is required to suck the brake fluid from the reservoir, however, a relatively high suction performance is needed. The pump having such high suction performance is so costly that the cost of the system will be raised as a whole, especially in the case where the wheel cylinders in more than two circuits must be pressurized by the pump provided in each circuit.